Yeast compositions for converting bio-available nitrogen in a culture medium to intracellular nitrogen

ABSTRACT

Compositions comprising a plurality of yeast cells, wherein said plurality of yeast cells have been cultured in the presence of an alternating electric field having a specific frequency and a specific field strength for a period of time sufficient to substantially increase the capability of said plurality of yeast cells to convert bio-available nitrogen in a culture medium into their own biomass. Also included are methods of making such compositions.

FIELD OF THE INVENTION

The invention relates to the use of yeast cells that are highly efficient in incorporating biologically available nitrogen in a culture medium into their own biomass. These yeasts are useful in waste treatment, and can be obtained by growth in electromagnetic fields with specific frequencies and field strengths.

BACKGROUND OF THE INVENTION

Environmental pollution by urban sewage and industrial waste water has posed a serious health threat to living organisms in the world. Currently, the most common methods for large-scale water treatment include the activated sludge technology and the biomembrane technology. These technologies rely on the innate abilities of myriad natural microorganisms, such as fungi, bacteria and protozoa, to degrade pollutants. However, the compositions of these natural microbial components are difficult to control, affecting the reproducibility and quality of water treatment. Moreover, pathogenic microbes existing in these activated sludge or biomembranes cannot be selectively inhibited, and such microbes usually enter the environment with the treated water, causing “secondary pollution.”

Further, most of the current technologies cannot degrade harmful chemicals such as pesticides, insecticides, and chemical fertilizers. These technologies also cannot alleviate eutrophication, another serious environmental problem around the world. Eutrophication is usually caused by sewage, industrial waste water, fertilizers and the like. It refers to waters (e.g., a lake or pond) rich in mineral and organic nutrients that promote a proliferation of plant life, especially algae, which reduces the dissolved oxygen content or otherwise deteriorates water quality. Eutrophication often results in the extinction of other organisms.

SUMMARY OF THE INVENTION

This invention is based on the discovery that certain yeast cells can be activated by electromagnetic fields having specific frequencies and field strengths to convert biologically available nitrogen, a major environmental pollutant, to intracellular nitrogen (i.e., the yeast cells incorporate biologically available nitrogen in their environs into their own biomass). Compositions comprising these activated yeast cells can therefore be useful for waste treatment, for example, for treatment of sewage, industrial waste water, surface water, drinking water, sediment, soil, garbage, and manure, to reduce the content of available nitrogen in the waste. Waste treatment methods using these compositions are more effective, efficient and economical in preventing eutrophication than conventional methods.

This invention includes a composition comprising a plurality of yeast cells that have been cultured in an alternating electric field having a frequency in the range of about 660 to 680 MHz (e.g, 662, 664, 666, 668, 670, 672, 674, 676, or 678 MHz) and a field strength in the range of about 0.1 to 350 mV/cm (e.g., 140-320 or 120-290 mV/cm). The yeast cells are cultured for a period of time sufficient to substantially increase the capability of said plurality of yeast cells to convert biologically available nitrogen in a culture medium into intracellular nitrogen. In one embodiment, the frequency and/or the field strength of the alternating electric field can be altered within the aforementioned ranges during said period of time. In other words, the yeast cells can be exposed to a series of electromagnetic fields. An exemplary period of time is about 12-420 hours (e.g., 192-304 or 226-412 hours).

This invention also includes a composition comprising a plurality of yeast cells that have been cultured in an alternating electric field having a frequency in the range of about 2160 to 2190 MHz (e.g; 2170 to 2185 MHz) and a field strength in the range of about 0.1 to 350 mV/cm (e.g., 140-320 mV/cm) for a period of time sufficient to substantially increase the capability of said plurality of yeast cells to convert ammonium in a culture medium into intracellular nitrogen.

Yeast cells that can be included in this composition can all be obtained from the China General Microbiological Culture Collection Center (“CGMCC”), a depository recognized under the Budapest Treaty (China Committee for Culture Collection of Microorganisms, Institute of Microbiology, Chinese Academy of Sciences, Haidian, P.O. BOX 2714, Beijing, 100080, China). Useful yeast species include, but are not limited to, Saccharomyces cerevisiae, Candida tropicalis and Geotrichum candidum. For instance, the yeast cells can be of the strain Saccharomyces cerevisiae AS2.196, AS2.336, AS2.400, AS2.416, AS2.423, or AS2.982; Saccharomyces willianus Saccardo AS2.152 or AS2.614; Candida tropicalis AS2.1387; or Geotrichum candidum AS2.498.

This invention also embraces a composition comprising a plurality of yeast cells, wherein said plurality of yeast cells have been activated such that they have a substantially increased capability to convert bio-available nitrogen in a culture medium into intracellular nitrogen as compared to unactivated yeast cells.

As used herein, “biologically available,” “bio-available,” or “biologically assimilable” nitrogen refers to nitrogen that is readily available, useable, or assimilable by living organisms for survival and/or growth. Exemplary bio-available or bio-assimilable nitrogen includes, but is not limited to, NH₄ ⁺, N₃ ⁻ and NO₂ ⁻, other water-soluble inorganic nitrogen-containing compounds, and organic nitrogen-containing compounds.

A “substantial increase” means an increase of more than 10 (e.g., 10², 10³, 10⁴, 10⁵, or 10⁶) fold.

A “culture medium” refers to a medium used in a laboratory for selecting and growing a given yeast strain, or to liquid or solid waste in need of treatment.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Exemplary methods and materials are described below, although methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention. All publications and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. The materials, methods, and examples are illustrative only and not intended to be limiting.

Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an exemplary apparatus for activating yeast cells using electromagnetic fields. 1: yeast culture; 2: container; 3: power supply.

DETAILED DESCRIPTION OF THE INVENTION

This invention is based on the discovery that certain yeast strains can be activated by electromagnetic fields (“EMF”) having specific frequencies and field strengths to become highly efficient in converting bio-available nitrogen in a culture medium to intracellular nitrogen. Yeast cells having this function are defined herein as belonging to the same “functional group.” Bio-available nitrogen causes undesired eutrophication of water bodies around the world. Thus, compositions containing the activated yeast cells can be used in waste treatment to reduce eutrophication.

Without being bound by any theory or mechanism, the inventor believes that EMFs activate or enhance the expression of a gene or a set of genes in yeast cells such that the yeast cells become active or more efficient in performing certain metabolic activities which lead to the desired nitrogen conversion result.

I. Yeast Strains Useful in the Invention

The types of yeasts useful in this invention include, but are not limited to, yeasts of the genera of Saccharomyces, Schizosaccharomyces, Sporobolomyces, Torulopsis, Trichosporon, Wickerhamia, Ashbya, Blastomyces, Candida, Citeromyces, Crebrothecium, Cryptococcus, Debaryomyces, Endomycopsis, Eremothecium, Geotrichum, Hansenula, Kloeckera, Lipomyces, Pichia, Rhodosporidium, and Rhodotorula.

Exemplary species within the above-listed genera include, but are not limited to, Saccharomyces cerevisiae, Saccharomyces bailii, Saccharomyces carlsbergensis, Saccharomyces chevalieri, Saccharomyces delbrueckii, Saccharomyces exiguus, Saccharomyces fermentati, Saccharomyces logos, Saccharomyces mellis, Saccharomyces microellipsoides, Saccharomyces oviformis, Saccharomyces rosei, Saccharomyces rouxii, Saccharomyces sake, Saccharomyces uvarum, Saccharomyces willianus, Saccharomyces sp., Saccharomyces ludwigii, Saccharomyces sinenses, Saccharomyces bailii, Saccharomyces carlsbergensis, Schizosaccharomyces octosporus, Schizosaccharomyces pombe, Sporobolomyces roseus, Sporobolomyces salmonicolor, Torulopsis candida, Torulopsis famta, Torulopsis globosa, Torulopsis inconspicua, Trichosporon behrendoo, Trichosporon capitatum, Trichosporon cutaneum, Wickerhamia fluoresens, Ashbya gossypii, Blastomyces dermatitidis, Candida albicans, Candida arborea, Candida guilliermondii, Candida krusei, Candida lambica, Candida lipolytica, Candida parakrusei, Candida parapsilosis, Candida pseudotropicalis, Candida pulcherrima, Candida robusta, Candida rugousa, Candida tropicalis, Candida utilis, Citeromyces matritensis, Crebrothecium ashbyii, Cryptococcus laurentii, Cryptococcus neoformans, Debaryomyces hansenii, Debaryomyces kloeckeri, Debaryomyces sp., Endomycopsis fibuligera, Eremothecium ashbyii, Geotrichum candidum, Geotrichum ludwigii, Geotrichum robustum, Geotrichum suaveolens, Hansenula anomala, Hansenula arabitolgens, Hansenula jadinii, Hansenula saturnus, Hansenula schneggii, Hansenula subpelliculosa, Kloeckera apiculata, Lipomyces starkeyi, Pichiafarinosa, Pichia membranaefaciens, Rhodosporidium toruloides, Rhodotorula aurantiaca, Rhodotorula glutinis, Rhodotorula minuta, Rhodotorula rubar, and Rhodotorula sinesis.

Yeast strains useful for this invention can be obtained from laboratory cultures, or from publically accessible culture depositories, such as CGMCC and the American Type Culture Collection, 10801 University Boulevard, Manassas, Va. 20110-2209. Non-limiting examples of useful strains (with accession numbers from CGMCC) are Saccharomyces cerevisiae AS2.196, AS2.336, AS2.400, AS2.416, AS2.423 and AS2.982; Saccharomyces willianus Saccardo AS2.152 and AS2.614; Candida tropicals AS2.1387; and Geotrichum candidum AS2.498.

Although it is preferred, the preparation of the yeast compositions of this invention is not limited to starting with a pure strain of yeast. A yeast composition of the invention may be produced by culturing a mixture of yeast cells of different species or strains that have the same function. The ability of any species or strain of yeast to perform this function can be readily tested by methods known in the art. See also discussions below.

Certain yeast species that can be activated according to the present invention are known to be pathogenic to human and/or other living organisms. These yeast species include, for example, Ashbya gossypii, Blastomyces dermatitidis, Candida albicans, Candida parakrusei, Candida tropicalis, Citeromyces matritensis, Crebrothecium ashbyii, Cryptococcus laurentii, Cryptococcus neoformans, Debaryomyces hansenii, Debaryomyces kloeckeri, Debaryomyces sp., and Endomycopsis fibuligera. Under certain circumstances, it may be less preferable to use such pathogenic yeasts in this invention. If use of these species is necessary, caution should be exercised to minimize the leak of the yeast cells into the final treatment product that enters the environment.

I. Application of Electromagnetic Fields

An electromagnetic field usefull in this invention can be generated and applied by various means well known in the art. For instance, the EMF can be generated by applying an alternating electric field or an oscillating magnetic field.

Alternating electric fields can be applied to cell cultures through electrodes in direct contact with the culture medium, or through electromagnetic induction. See, e.g., FIG. 1. Relatively high electric fields in the medium can be generated using a method in which the electrodes are in contact with the medium. Care must be taken to prevent electrolysis at the electrodes from introducing undesired ions into the culture and to prevent contact resistance, bubbles, or other features of electrolysis from dropping the field level below that intended. Electrodes should be matched to their environment, for example, using Ag-AgCl electrodes in solutions rich in chloride ions, and run at as low a voltage as possible. For general review, see Goodman et al., Effects of EMF on Molecules and Cells, International Review of Cytology, A Survey of Cell Biology, Vol. 158, Academic Press, 1995.

The EMFs useful in this invention can also be generated by applying an oscillating magnetic field. An oscillating magnetic field can be generated by oscillating electric currents going through Helmholtz coils. Such a magnetic field in turn induces an electric field.

The frequencies of EMFs useful in this invention range from about 5 MHz to 5000 MHz, e.g., from about 600 to 700 MHz (e.g., 660 to 680 MHz) or 2160 to 2190 MHz (e.g., 2170 to 2185 MHz). Exemplary frequencies are 662, 664, 666, 668, 670, 672, 674, 676, 678, 2170, 2171, 2172, 2173, 2174, 2175, 2176, 2177, 2178, 2179, 2180, 2181, 2182, 2183, 2184, and 2185 MHz. The field strength of the electric field useful in this invention ranges from about 0.1 to 350 mV/cm, e.g., from 100-350 mV/cm (e.g., 140-320 or 120-290 mV/cm). Exemplary field strengths are 126, 152, 196 and 310 mV/cm.

When a series of EMFs are applied to a yeast culture, the yeast culture can remain in the same container while the same set of EMF generator and emitters is used to change the frequency and/or field strength. The EMFs in the series can each have a different frequency or a different field strength; or a different frequency and a different field strength. Such frequencies and field strengths are preferably within the above-described ranges. In one embodiment, an EMF at the beginning of the series has a field strength identical to or lower than that of a subsequent EMF, such that the yeast cell culture is exposed to EMFs of progressively increasing field strength. Although any practical number of EMFs can be used in a series, it may be preferred that the yeast culture be exposed to a total of 2, 3, 4, 5, 6, 7, 8, 9 or 10 EMFs in a series.

By way of example, the yeast cells can be cultured in a first series of alternating electric fields each having a frequency in the range of 660 to 680 MHz and a field strength in the range of 100 to 350 mV/cm. The yeast cells are exposed to each ENF for about 18 to 25 hours. After culturing in the first series of EMFs, the resultant yeast cells are further incubated in a second series of alternating electric fields for a total of 25 to 130 hours. It may be preferred that the electric field frequencies of the second series are identical to those of the first series in sequence while the field strengths in the second series are increased to a higher level within the range of 100 to 350 mV/cm.

In another embodiment, the yeast cells can be cultured in a first series of alternating electric fields each having a frequency in the range of 2170 to 2185 MHz and a field strength in the range of 140 to 320 mV/cm. The yeast cells are exposed to each EMF for about 18 to 25 hours. After culturing in the first series of EMF, the resultant yeast cells are further incubated in a second series of alternating electric fields for a total of 25 to 130 hours. It may be preferred that the frequencies in the second series of alternating electric fields are identical to those of the first series in sequence while the field strengths in the second series are increased to a higher level within the range of 140 to 320 mV/cm.

Although the yeast cells can be activated after even a few hours of culturing in the presence of an EMF, it may be preferred that the activated yeast cells be allowed to multiply and grow in the presence of the EMF(s) for a total of 192-304 or 226-412 hours.

FIG. 1 illustrates an exemplary apparatus for generating alternating electric fields. An electric field of a desired frequency and intensity is generated by an AC source (3) capable of generating an alternating electric field, preferably in a sinusoidal wave form, in the frequency range of 5 to 5000 MHz. Signal generators capable of generating signals with a narrower frequency range can also be used. If desirable, a signal amplifier can also be used to increase the output. The alternating electric field can be applied to the culture by a variety of means including placing the yeast culture in close proximity to the signal emitters. In one embodiment, the electric field is applied by electrodes submerged in the culture (1). In this embodiment, one of the electrodes can be a metal plate placed on the bottom of the container (2), and the other electrode can comprise a plurality of electrode wires evenly distributed in the culture (1) so as to achieve even distribution of the electric field energy. The number of electrode wires used depends on the volume of the culture as well as the diameter of the wires. In a preferred embodiment, for a culture having a volume up to 5000 ml, one electrode wire having a diameter of 0.1 to 1.2 mm can be used for each 100 ml of culture. For a culture having a volume greater than 1000 L, one electrode wire having a diameter of 3 to 30 mm can be used for each 1000 L of culture.

II. Culture Media

Culture media useful in this invention contain sources of nutrients assimilable by yeast cells. In this invention, a culture medium refers to a laboratory culture medium, or the liquid or solid waste in need of treatment. Complex carbon-containing substances in a suitable form (e.g., carbohydrates such as sucrose, glucose, dextrose, maltose and xylose; or coal) can be the carbon sources for yeast cells. In a laboratory culture medium, the exact quantity of the carbon sources can be adjusted in accordance with the other ingredients of the medium. In general, the amount of carbohydrate varies between about 0.1% and 5% by weight of the medium and preferably between about 0.1% and 2%, and most preferably about 1%. These carbon sources can be used individually or in combination. Among the inorganic salts which can be added to a laboratory culture medium are the customary salts capable of yielding sodium, potassium, calcium, phosphate, sulfate, carbonate, and like ions. Non-limiting examples of nutrient inorganic salts are (NH₄)₂HPO₄, CaCO₃, KH₂PO₄, MgSO₄, NaCl, and CaSO₄.

III. Electromagnetic Activation of Yeast Cells

To activate the innate ability of yeast cells to convert nitrogen, the cells can be cultured in an appropriate medium under sterile conditions at 25° C.-30° C. (e.g., 28° C.) for a sufficient amount of time, e.g., 12-420 hours (e.g., 192-304 or 226-412 hours), in an alternating electric field or a series of alternating electric fields as described above. An exemplary medium contains in per 1000 ml of sterile water: 12 g of sucrose, 4 g of NH₄NO₂, 0.25 g of NaCl, 0.25 g of MgSO₄.7H₂O, 3.5 g of CaCO₃.5H₂O, 0.5 g of CaSO₄.2H₂O, and 0.2 g of K₂HPO₄. The culturin process may preferably be conducted under conditions in which the concentration of dissolved oxygen is between 0.025 to 0.8 mol/m³, preferably 0.4 mol/m³. The oxygen level can be controlled by, for example, stirring and/or bubbling.

Subsequently, the yeast cells can be measured for their ability to convert available nitrogen to intracellular nitrogen using standard methods. In an exemplary method, waste water from a nitrogen fertilizer manufacturer containing high levels of ammonium salts and/or nitrate or nitride salts is mixed with distilled water to achieve the following COD concentrations: (1) 100-1,000 mg/L; (2) 1,000-5,000 mg/L; (3) 5,000-10,000 mg/L; and (4) 10,000-50,000 mg/L. The solutions are then inoculated with dry yeast cell preparation, at a concentration of 0.2-0.6 g/L, and cultured for 24-48 hours at 10-40° C. The COD levels of the solutions are then measured using standard techniques. The difference between the COD levels before and after 24-48 hours indicates the nitrogen-converting activity of the yeast cells. Another method for determining nitrogen compound levels in a culture medium is described below in the working example.

Essentially the same protocol as described above can be used to grow activated yeast cells. To initiate the process, each 100 ml of culture medium is inoculated with the nitrogen-converting yeast cells at a density of 10²-10⁵ cells/ml, preferably 3×10²-10⁴ cells/ml. The culturing process is carried out at about 20-40° C., preferably at about 25-28° C., for 48-96 hours. The process can be scaled up or down according to needs. For an industrial scale of production, seventy-five liters of a sterile culture medium are inoculated with the yeast cells. This culture medium consists of 10 L of the culture medium described above for this particular yeast functional group, 30 kg of starch, and 65 L of distilled water. At the end of the culturing process, the yeast cells may preferably reach a concentration of 2×10¹⁰ cells/ml. The cells are recovered from the culture by various methods known in the art, and stored at about 15-20° C. The yeast should be dried within 24 hours and stored in powder form.

IV. Acclimatization of Yeast Cells To Waste Environment

In yet another embodiment of the invention, the yeast cells may also be cultured under certain conditions so as to acclimatize the cells to a particular type of waste. This culturing process results in better growth and survival of the yeasts in a particular waste environment.

To do this, the yeast cells can be mixed with waste water from a particular source at 10⁶ to 10⁸ cells (e.g., 10⁷ cells) per 1000 ml. The yeast cells are then exposed to an alternating electric field as described above. The strength of the electric field is about 100 to 400 mV/cm (e.g., 120-250 mV/cm). The culture is incubated at temperatures that cycle between about 5° C. to about 45° C. at a 5° C. increment. For example, in a typical cycle, the temperature of the culture may start at 5° C. and be kept at this temperature for about 1-2 hours, then adjusted up to 10° C. and kept at this temperature for 1-2 hours, then adjusted to 15° C. and kept at this temperature for about 1-2 hours, and so on and so forth, until the temperature reaches 45° C. Then the temperature is brought down to 40° C. and kept at this temperature for about 1-2 hours, and then to 35° C. and kept at this temperature for about 1-2 hours, and so on and so forth, until the temperature returns to 5° C. The cycles are repeated for about 48-96 hours. The resulting yeast cells are then dried and stored at 0-4° C.

V. Manufacture of the Waste Treatment Compositions

The yeast cells of the invention can be mixed with an appropriate filler, such as rock powder and coal ash at the following ratio: 600 L of mixed yeast cell culture at 2×10¹⁰ cells/ml and 760 kg of filler materials. The mixture is quickly dried at a temperature below 65° C. for 10 minutes in a dryer, and then further dried at a temperature below 70° C. for no more than 30 minutes, so that the water content is less than 7%. The dried composition is then cooled to room temperature for packaging.

These dried yeast compositions may be used to treat polluted surface water, sewage, or any other type of solid or liquid waste. By way of example, to treat polluted surface water, a yeast solution may be prepared by adding 1 kg of the dried yeast composition to 30 L of clean water. The yeast solution is then sprayed onto the polluted surface water at about 1-3 L of the solution per square meter of the polluted surface water. To treat sewage or any other type of waste water, a yeast solution may be prepared by adding about 1 kg of the dried yeast composition to 10-30 L of clean water. The yeast solution is incubated at 10-35° C. for 24-48 hours. The resultant yeast solution is then added to the waste water at about 3-20 L of the solution per liter of waste water.

VI. Examples

The following examples are meant to illustrate the methods and materials of the present invention. Suitable modifications and adaptations of the described conditions and parameters which are obvious to those skilled in the art are within the spirit and scope of the present invention.

EXAMPLE 1 Conversion of NH₄ ⁺

Saccharomyces cerevisiae Hansen AS2.614 were cultured in the presence of a series of alternating electric fields in the following sequence: the yeast cells were exposed to (1) an alternating electric field having a frequency of 662 MHz and a field strength of 152 mV/cm for 18 hours; (2) then to an alternating electric field having a frequency of 666 MHz and a field strength of 152 mV/cm for 18 hours; (3) then to an alternating electric field having a frequency of 672 MHz and a field strength of 152 mV/cm for 18 hours; (4) then to an alternating electric field having a frequency of 678 MHz and a field strength of 152 mV/cm for 18 hours; (5) then to an alternating electric field having a frequency of 662 MHz and a field strength of 310 mV/cm for 25 hours; (6) then to an alternating electric field having a frequency of 666 MHz and a field strength of 310 mV/cm for 25 hours; (7) then to an alternating electric field having a frequency of 672 MHz and a field strength of 310 mV/cm for 35 hours; and (8) finally to an alternating electric field having a frequency of 678 MHz and a field strength of 310 mV/cm for 35 hours.

To test the ability of the EMF-treated AS2.614 cells to convert extracellular chemicals containing the NH₄ ⁺ ion into the cells' own biomass, waste water was supplemented with ammonium sulfate to reconstitute a solution containing ammonium sulfate at 200 mg/L. 0.1 ml of the EMF-treated AS2.614 cells at a concentration higher than 10⁸ cells/ml was added to 100 L of the ammonium sulfate solution and cultured at 28° C. for 48 hours (solution A). One hundred liters of the ammonium sulfate solution containing the same number of non-treated cells (solution B) or containing no cells (solution C) were used as controls. After 48 hours of incubation, the ammonium sulfate solutions were examined using Kjeldahl nitrogen-fixation test. The results showed that after 48 hours of incubation, the anunonium sulfate concentration of solution A decreased more than 22% relative to solution C. In contrast, the ammonium sulfate concentration of solution B showed no significant change relative to solution C.

EXAMPLE 2 Conversion of Nitrates and Nitrites

Saccharomyces cerevisiae Hansen AS2.982 were cultured in the presence of a series of alternating electric fields in the following sequence: the yeast cells were exposed to (1) an alternating electric field having a frequency of 661 MHz and a field strength of 126 mV/cm for 25 hours; (2) then to an alternating electric field having a frequency of 665 MHz and a field strength of 126 mV/cm for 25 hours; (3) then to an alternating electric field having a frequency of 672 MHz and a field strength of 126 mV/cm for 25 hours; (4) then to an alternating electric field having a frequency of 676 MHz and a field strength of 126 mV/cm for 25 hours; (5) then to an alternating electric field having a frequency of 661 MHz and a field strength of 196 mV/cm for 25 hours; (6) then to an alternating electric field having a frequency of 665 MHz and a field strength of 196 mV/cm for 25 hours; (7) then to an alternating electric field having a frequency of 672 MHz and a field strength of 196 mV/cm for 38 hours; and (8) finally to an alternating electric field having a frequency of 676 MHz and a field strength of 196 mV/cm for 38 hours.

To test the ability of the EMF-treated AS2.982 cells to convert extracellular nitrates and/or nitrites into the cells' own biomass, waste water was supplemented with sodium nitrate and sodium nitrite to reconstitute a solution containing sodium nitrate/sodium nitrite at a total concentration of 200 mg/L. 0.1 ml of the EMF-treated AS2.982 cells at a concentration higher than 1¹⁰ cells/ml was added to 100 L of the sodium nitrate/sodium nitrite solution and cultured at 28° C. for 48 hours (solution A). One hundred liters of the nitrate/nitrite solution containing the same number of non-treated cells (solution B) or containing no cells (solution C) were used as controls. After 48 hours of incubation, the nitrate/nitrite solutions were examined using Kjeldahl nitrogen-fixation test. The results showed that after 48 hours of incubation, the nitrate/nitrite concentration of solution A decreased more than 29% relative to solution C. In contrast, the nitrate/nitrite concentration of solution B showed no significant change relative to solution C.

While a number of embodiments of this invention have been set forth, it is apparent that the basic constructions may be altered to provide other embodiments which utilize the compositions and methods of this invention. 

What is claimed is:
 1. A composition comprising a plurality of yeast cells, wherein said plurality of yeast cells are characterized by a substantial increase in their capability to convert bio-available nitrogen in a culture medium into intracellular nitrogen as a result of having been cultured in the presence of an alternating electric field having a frequency in the range of 660 to 680 MHz and a field strength in the range of 0.1 to 350 mV/cm, as compared to yeast cells not having been so cultured.
 2. The composition of claim 1, wherein said field strength is in the range of 100 to 350 mV/cm.
 3. The composition of claim 1, wherein said yeast cells are cells of the species Saccharomyces cerevisiae, Saccharomyces willianus, Candida tropicalis or Geotrichum candidum.
 4. The composition of claim 1, wherein said yeast cells are derived from cells of the strain deposited at The China General Microbiological Culture Collection Center with an accession number selected from the group consisting of AS2.152, AS2.196, AS2.336, AS2.400, AS2.416, AS2.423, AS2.498, AS2.614, AS2.982, and AS2.1387.
 5. The composition of claim 1, wherein the bio-available nitrogen is ammonium.
 6. The composition of claim 5, wherein said frequency is in the range of 660 to 680 MHz and said field strength is in the range of 140-320 mV/cm.
 7. The composition of claim 1, wherein the bio-available nitrogen is nitrate or nitrite.
 8. The composition of claim 7, wherein said frequency is in the range of 660 to 680 MHz and said field strength is in the range of 120-290 mV/cm.
 9. A composition comprising a plurality of yeast cells, wherein said plurality of yeast cells are characterized by a substantial increase in their capability to convert ammonium in a culture medium into intracellular nitrogen as a result of having.
 10. A method of preparing a yeast composition, comprising culturing a plurality of yeast cells in the presence of an alternating electric been cultured in the presence of an alternating electric field having a frequency in the range of 2160 to 2190 MHz and a field strength in the range of 140 to 320 mV/cm, as compared to yeast cells not having been so cultured. 